Many shallow lakes are over-fertilized with nutrients. This favors algae, clouds the water and endangers the entire ecosystem. So far, aquatic researchers have tried to avert this fate by introducing biological measures against the algal blooms. However, in the long term, the lake ecosystem still falls back to its murky initial state, as scientists have now discovered. The only solution: significantly reduce the nutrients introduced.
Sewage treatment plants, agriculture and other human influences ensure that water bodies around the world are over-fertilized with nutrients such as phosphorus or nitrogen. Until now, hydrologists had assumed that shallow water lakes can react to this over-fertilization in two ways. Either with clear water and numerous aquatic plants or with cloudy water and abundance of algae. The states can also alternate, but both are of a long-term nature, according to the theory.
Algae out and everything is fine?
The logical consequence: In order to make a lake plagued by algal blooms “healthy” again, it is enough to take measures against the algae. Then the lake takes on its second possible state with clear water and stays in it for a long time – despite the continued high concentration of nutrients. Based on this basic idea, measures have already been adopted several hundred times to improve the condition of turbid lakes. One of these measures is to put predatory fish into the lake in a targeted manner. There they should eat other fish, which in turn can then eat fewer small crustaceans. The crabs, in turn, consume more algae overall, which reduces algae growth. Finally, without an algal bloom, the risk of oxygen depletion and toxins in the water and the resulting fish kills decreases.
“That somehow sounds too good to be true,” says Daniel Graeber from the Helmholtz Center for Environmental Research in Leipzig. He is part of a research team that has now subjected this theory to a reality check. To do this, the scientists analyzed long-term monitoring data from a total of 902 shallow water lakes in Denmark and the USA. They investigated how the relationship between nutrient concentration and algae growth evolved over time. With the help of a special statistical method, they were finally able to check whether the lakes can actually only assume two alternative states and how stable and self-sustaining they are.
The nutrient input must decrease
The result: Algae growth was inevitable in all over-fertilized lakes. “None of the lakes showed a different response to high nutrient concentrations. The explanatory model of the two alternative stable states does not appear to exist in reality – at least for lakes in temperate latitudes,” explains Graeber. However, this does not mean that the measures taken so far have been completely useless. The researchers were able to determine that the fight against algae had made the lakes clearer again and ensured that aquatic plants settled there.
However, these successes were short-lived. After five to ten years at the latest, the lakes had returned to their cloudy original state. In practice, this means: “Biomanipulative measures such as stocking with predatory fish cannot stabilize the shallow-water lake ecosystem in the long term. There is no alternative to the only way to keep the balance of shallow lakes in a permanently stable state: nutrient inputs must be consistently reduced,” says Graeber.
Source: Helmholtz Center for Environmental Research (UFZ); Specialist article: Nature Communications,
doi: 10.1038/s41467-023-36043-9